Seed priming and foliar application of GA3 enhance disease control in tomato (Solanum lycopersicum L.)

The research was conducted at Incheck Farm, Kalyani, Nadia, West Bengal, India, during November to March of 2019–2020 and 2020–2021. The treatment consisted of the tomato genotype BCT-25, sourced from the All India Coordinated Research Project (AICRP) on Vegetable Crops at Bidhan Chandra Krishi Viswavidyalaya, Nadia, West Bengal, India. After standardising the concentration and duration, various chemicals were used for seed priming, with dry seeds serving as the control (Table 1).

These treatments were evaluated with and without gibberellic acid (GA). Treated seeds were immediately broadcasted in separate nursery beds measuring 1 m × 1 m after priming. Standard agronomic practices were followed for raising seedlings in individual plots. At 21 days after sowing (DAS), the seedlings were transplanted into main plots (3 m × 1.2 m) with a spacing of 60 cm × 60 cm, which were arranged in a strip plot design with three replications. Each block or replication was further divided into 10 horizontal strips and 2 vertical strips.

The recommended fertiliser was applied to the main plots at a rate of N:P:K = 150:75:75 kg · ha⁻¹. Foliar application of 100 ppm GA₃ was applied 25 days after transplanting (Ray and Bordolui, 2020). Observations were recorded from standing crops to assess disease incidence and severity under natural conditions during November–March of 2019–2020 and 2020–2021.

The incidences of collar rot and bacterial wilt diseases were calculated as follows: Percent disease incidence =(Number of plants infectedTotal number of plants)×100{\rm{Percent disease incidence }} = \left( {{{{\rm{Number of plants infected}}} \over {{\rm{Total number of plants}}}}} \right) \times 100

The severity of different foliar diseases (target leaf spot, early blight, late blight and tomato leaf curl viral disease) was recorded from the entire individual plot via visual observation on different disease grading scales. Ten plants were selected randomly and tagged from each treatment to assess the disease severity from transplanting to harvest at 15-day intervals. Disease severity was calculated by using a 0–5 scale for target leaf spot and early blight (Mayee and Datar, 1986), late blight by a 0–5 scale (Akhtar et al., 2012) and tomato leaf curl viral disease by using a 0–4 scale (Friedmann et al., 1988). The percent disease index (PDI) was calculated as follows (McKinney, 1923): Percent disease index =(Sum of numerical ratingsTotal number of observations)×(100Maximum disease score){\rm{Percent disease index }} = \left( {{{{\rm{Sum of numerical ratings}}} \over {{\rm{Total number of observations}}}}} \right) \times \left( {{{100} \over {{\rm{Maximum disease score}}}}} \right)

This research was conducted via a two-factor strip plot design. All the analyses included in the manuscript were performed via OPSTAT version 1.0.2 (Sheoran et al., 1998), including angular transformation.

Compared with the control, all priming materials in combination with GA₃ and without GA₃ decreased bacterial wilt disease in tomatoes (Figure 1 and Figure S1 in Supplementary Materials) during both seasons. In the second-year trial, the disease incidence was slightly higher than that in the first year for all the treatments. Compared with the seed priming treatments, the exogenous application of GA₃ significantly influenced the incidence of bacterial wilt disease in tomatoes, and G₁ (15.96%, 17.45%) presented a lower disease incidence than did G₀ (17.44%, 18.64%) during both years (Table 2). On average, with and without GA₃, T₈ (12.79%, 14.08%) had the minimum incidence, followed by T₃ and T₁ during the first and second years, although T₃ and T₁ were not significantly different from each other; it was the maximum for T₀ (21.22%, 22.49%) in both years. During both years, T₈, that is, KH₂PO₄ in combination with GA₃ (11.98%, 13.63%) and without GA₃ (13.60%, 14.52%), had the lowest disease incidence, followed by T₃ and T₁, although a statistically non-significant influence was observed for the performance of priming agents with and without GA₃ (Table 2).

Figure 1.

Bacterial wilt disease.

There was no significant difference among the treatments with the application of foliar application of GA₃ to individual priming agents in either season. Disease reduction was also calculated for the control. The data revealed that the greatest degree of disease control was achieved with the combination of GA₃ and without GA₃ application during both years. The data revealed that the greatest degree of disease control was observed in T₈ in combination with GA₃ and without GA₃ application during both years.

The application of GA₃ as a foliar spray had a significant influence on the incidence of collar rot disease (Figure 2 and Figure S1 in Supplementary Materials) in tomatoes; during both seasons, the results revealed that G₁ had a lower disease incidence (11.20% and 12.51%) than did G₀ (12.53% and 13.82%) when the mean was made over seed priming. The trait varied significantly because of seed priming, with an average over with and without GA₃ application; minimum disease incidence was observed for T₃, that is, 10% polyethylene glycol (PEG) (8.68% and 10.04%) during both years, followed by T₈ and T₁; maximum disease incidence was observed after the control, that is, T₀ (14.78% and 16.18%) in 2 years (Table 3). Non-significant variation was observed for the influence of the seed priming agent either alone or in combination with GA₃ and for the influence of foliar GA₃ application on individual priming materials. The disease reduction was also calculated for the control (Table 3). The data revealed that the greatest degree of disease control, that is, PEG, was observed in T₃ during both years.

Figure 2.

Collar rot disease.

In the case of target leaf spot disease in tomato (Figure 3 and Figure S2 in Supplementary Materials), all priming materials in combination with GA₃ and without GA₃ decreased the incidence of disease compared with the control in both seasons. The disease severity percentage was slightly greater from 2020 to 2021 than from 2019 to 2020 for all the treatments (Table 4). Foliar application of GA₃ significantly influenced the disease severity percentage or PDI when the average was greater than that of the seed priming treatments; G₁ (30.83%, 32.21%) presented a lower disease severity than did G₀ (33.11%, 34.48%) during both years. On average, with and without GA₃, treatment with 2% KH₂PO₄ (25.79%, 27.27%) resulted in the lowest degree of disease severity, followed by KNO₃ and the moringa leaf extract. The lowest PDI was obtained from 2% KH₂PO₄ (24.70%, 26.87% and 26.22%, 28.31%), followed by 5% KNO₃ and moringa leaf extract in combination with GA₃ and without GA₃ for both seasons. The highest disease severity was observed in the control (36.64%, 38.73% and 37.95%, 40.11%, respectively) (Table 4).

Figure 3.

Target leaf spot disease.

Disease reduction was also calculated for the control (Table 4). The data revealed that the greatest degree of disease control was observed. 2% KH₂PO₄, followed by 5% KNO₃ and moringa leaf extract in combination with GA₃ and without GA₃ for both seasons.

Compared with the control, all priming materials in combination with GA₃ and without GA₃ decreased early blight disease (Figure 4 and Figure S2 in Supplementary Materials) during both seasons. In the first-year trial, the disease severity percentage was slightly lower than that in the second year in all the treatments. Significant variation was observed for the percent disease severity or PDI in the early blight stage of tomato plants after they were sprayed with GA₃, when the average was greater than that in the seed priming treatment group; G₁ (23.93%, 25.19%) presented a lower disease severity than did G₀ (25.23%, 26.58%) during both years (Table 5). On average, with and without GA₃, 2% KH₂PO₄ (18.61%, 19.91%) had the lowest disease severity, followed by 5% KNO₃ and moringa leaf extract in two consecutive seasons; the disease severity was greatest for T₀ (29.74%, 31.17%) in both years. During both seasons, 2% KH₂PO₄ (17.27%, 19.94% and 18.63%, 21.18%) in combination with GA₃ and without GA₃ had the lowest disease severity percentage or PDI, followed by 5% KNO₃ Moringa leaf extract.

Figure 4.

Early leaf blight disease.

The disease severity was greatest in the control (28.76%, 30.72%, and 30.08%, 32.26%, respectively) treatment. Disease reduction was also calculated for the control (Table 5). The data revealed that the greatest degree of disease control was associated with 2% KH₂PO₄, 5% KNO₃ and Moringa leaf extract in combination with GA₃ and without GA₃ was used for both seasons (Table 5).

While observing the influence of all priming agents in combination with GA₃ and without GA₃, a reduction in late blight disease in tomato (Figure 5 and Figure S2 in Supplementary Materials) was noted over the control in both seasons.

Figure 5.

Late blight disease.

Significant variation was noted for the percent disease severity or PDI after GA₃ application when the average was performed over the seed priming treatments; disease severity was lower in the treatments with GA₃ (20.10%, 21.35%) than in the treatments without GA₃ (21.05%, 22.40%) during both years (Figure 5). Among the seed priming treatments, 2% KH₂PO₄ (15.74%, 17.08%) resulted in the lowest disease severity, followed by 5% KNO₃ and moringa leaf extract during both seasons. When the mean disease severity was greater than that of G (with and without GA₃), it was highest for the control (25.15%, 26.40%) in both years (Table 6). The minimum disease severity percentage or PDI was noted for 2% KH₂PO₄ (14.92%, 16.56% and 16.20%, 17.95%), followed by 5% KNO₃ and moringa leaf extract in combination with GA₃ and without GA₃ for both seasons. The maximum disease severity was found control (24.67%, 25.63%, and 25.87%, 26.92%, respectively) (Table 6). The reduction in disease incidence was also greater than that of the control. The maximum reduction was observed. 2% KH₂PO₄ followed by 5% KNO₃ and T₁, that is, moringa leaf extract in combination with GA₃ and without GA₃, for both seasons.

In the case of tomato leaf curl disease (Figure 6 and Figure S2 in Supplementary Materials), all priming materials in combination with GA₃ and without GA₃ decreased the incidence of disease compared with the control in both seasons. The percent disease severity or PDI for this disease significantly varied after foliar application of GA₃, with an average over seed priming treatments during both years; lower disease severity was detected with GA₃ (14.14%, 15.48%) than without GA₃ (15.84%, 17.15%). In terms of the performance of the seed priming agents, the average over with and without GA₃, the trait varied significantly in both years; 2% KH₂PO₄ (11.94%, 13.31%) had the lowest disease severity, followed by 10% PEG and moringa leaf extract during the year and, in the second year, that they interchanged, although T₁ and T₃ were not significantly different from each other in either season. T₀ (19.15%, 20.54%) was highest in both years. The lowest disease severity percentage or PDI was obtained at 2% KH₂PO₄ (10.71%, 13.17% and 12.06%, 14.57%) in combination with GA₃ and without GA₃ for both seasons; 2% KH₂PO₄ was followed by 10% PEG and Moringa leaf extract, with slight changes in their position over the years; the highest disease severity was observed in the control (18.51%, 19.78% and 19.91%, 21.18%, respectively) (Table 7).

Figure 6.

Tomato leaf curl viral disease.

The results clearly demonstrate that both GA₃ application and seed priming treatments contributed to a reduction in bacterial wilt and collar rot incidence, although the magnitude of their effects varied between diseases and years. In the case of bacterial wilt, priming treatments (particularly T₁, T₃, and T₅) were consistently more effective than GA₃ alone, while the significant T × G interaction highlighted that genotype-specific responses played an important role in disease suppression. Similarly, for collar rot, seed priming emerged as the major factor driving disease reduction, with GA₃ acting mainly as a supportive enhancer by improving plant vigour and tolerance rather than directly suppressing pathogen activity. The interaction effects in collar rot also indicated that certain genotypes performed better under specific priming treatments when combined with GA₃, suggesting a synergistic benefit in some cases. Overall, while both factors contributed, the priming materials were the principal drivers of disease suppression in this study, and GA₃ primarily complemented their effect by enhancing plant growth and resilience.

The present study demonstrated that seed priming with 10% PEG and 2% KH₂PO₄ effectively decreased the incidence and severity of several tomato diseases. Specifically, KH₂PO₄ substantially reduced bacterial wilt, while PEG was more effective in reducing collar rot incidence. Furthermore, the application of KH₂PO₄, KNO₃ and moringa leaf extract lowered the severity of target leaf spot, early blight and late blight, suggesting that these treatments can enhance plant defence mechanisms. Although GA₃ application led to a marginal reduction in disease severity, its effect was statistically comparable to that of the non-GA₃ treatments, indicating that seed priming had a more pronounced role in disease suppression than GA₃ supplementation.

Our findings are consistent with earlier reports highlighting the disease-suppressive potential of KH₂PO₄ and other salt-based compounds. For instance, Reuveni et al. (1994) observed a 79% reduction in disease incidence in roses treated with KH₂PO₄ in combination with KOH, K₂HPO₄ NaHCO₃ or topiramate, while Ehret et al. (2002) reported that foliar applications of KCl, MgSO₄ and K₂HPO₄ significantly reduced powdery mildew in tomato. Similarly, Ata et al. (2008) demonstrated that KH₂PO₄ application effectively suppressed rust disease in wheat, and Abo-Elyousr et al. (2022) reported that foliar sprays of KH₂PO₄, K₂HPO₄ and MgSO₄ reduced powdery mildew in sunflower with efficacy comparable to conventional fungicides. Moreover, Reuveni et al. (1996) found that regular foliar applications of K₂HPO₄ or KH₂PO₄ were highly effective in controlling powdery mildew in cucumber. These results support the proposition that phosphate-based compounds act as alternative eco-friendly disease management tools.

The role of GA₃ in disease reduction has also been documented across multiple crops. In grapevines, GA₃ applications at bloom significantly reduced Botrytis and sour rot, with greater efficacy than two specific fungicide applications (Spring and Viret, 2010). Hed et al. (2011) further confirmed GA₃-mediated reductions in bunch rot severity in Vignoles and Chardonnay grapes. Similarly, da Silva et al. (2018) reported that GA₃ treatment in grapevines decreased Botrytis cinerea infection and enhanced cluster and rachis length. Other studies have shown GA₃-induced suppression of foliar pathogens, such as Alternaria dauci in carrot (Santos et al., 2000). The black spot of persimmon, caused by Alternaria alternata, was reduced by the application of GA, which reduced the relative humidity around the infection site due to increased calyx erectness (Perez et al., 1995). Janisiewicz et al. (2005) and Percival et al. (2009) noted that foliar application of KH₂PO₄ may protect against various pathogens via several host-pathogen interactions. Dibasic potassium phosphate was applied as a foliar spray to evaluate its ability to induce resistance against rust disease in sugar beet caused by Uromyces betae, which reduces both disease incidence and severity, as observed by Ata et al. (2008). Khalifa and Thabet (2014) reported that spraying KH₂PO₄ suppressed leaf rust disease in the sensitive wheat cultivar Sids-1, as indicated by reduced disease severity. These results collectively suggest that GA₃ influences both plant architecture and disease development, though in the present study its effect was not superior to seed priming. Phosphate priming with KH₂PO₄ not only supplies readily available phosphorus that enhances plant vigour but also induces systemic resistance by activating defence-related enzymes, strengthening cell walls and modulating oxidative stress responses, thereby limiting pathogen establishment. On the other hand, GA₃ influences plant architecture by loosening cell walls and reducing cluster compactness, which lowers humidity within the canopy and diminishes pathogen colonisation; it is also known to interact with defence signalling pathways such as salicylic acid and jasmonic acid, leading to enhanced resistance. Together, these mechanisms provide a plausible basis for the reduced disease incidence observed in this study.

Despite these encouraging results, certain limitations warrant consideration. Environmental conditions such as temperature, humidity and soil type, which strongly influence disease progression, were not systematically monitored. Additionally, the absence of molecular validation, such as pathogen DNA quantification or biochemical markers of induced defence, restricts the extrapolation of these findings to diverse agroclimatic regions. Future research should therefore incorporate pathogen quantification, biochemical assays for plant defence responses, and detailed climatic data to strengthen the scientific basis for integrating seed priming and salt-based treatments into tomato disease management strategies.